The present invention generally relates to a liquid crystal optical switching device and, in particular, relates to such a switching device having a minimized internal light path.
The use of optical fibers as a telecommunication transmission medium has numerous advantages compared to existing telecommunication transmission media. For example, optical fibers can sustain signals having wider bandwidths and, hence, can convey larger quantities of information than existing media. Further, light waves are shorter than the conventional microwaves currently used in many existing telecommunication systems and thus a substantial reduction in the physical size of components is readily achievable. This size reduction further results in cost reductions for materials, packages and manufacturing. Still further, current optical fibers exhibit little or no electromagnetic or radio frequency radiation, thus there is no consequential impact on the surrounding environment.
To be viable, every telecommunication system must include some means for controllably redirecting a signal, or a portion thereof, to or from a transmission medium, or between one or more such media. In the case of an optical telecommunication system, the signal redirecting means is an optical switch. Conventionally, the majority of optical switches have been mechanical, although recently liquid crystal switches have been proposed.
In general, mechanical switches require relatively high driving power and are subject to wear, abrasion and fatigue. Further, mechanical switches are prone to failure after repeated use. In addition, since a rather small optical fiber is usually displaced from alignment with one port fiber into alignment with another port fiber, mechanical switches are expensive due to the very small tolerances required to ensure the proper alignments.
Advantageously, a liquid crystal optical switching device has no moving parts and is thus free from many of the drawbacks of mechanical switches. Liquid crystal optical switches have been discussed and described, inter alia, in U.S. Pat. Nos. 4,201,442 issued to McMahon et al. on May 6, 1980, 4,278,327 issued to McMahon et al. on July 14, 1981 and 4,385,799 issued to Soref on May 31, 1983. Therein, a variety of liquid crystal switches are discussed and described wherein optical fibers are attached to the angled side surfaces of a pair of trapezoidal prisms. The trapezoidal prisms are arranged with the bases thereof parallel and liquid crystal material is positioned therebetween.
Although the optical switches described in the above-referenced patents have some advantages over mechanical switches, i.e. no moving parts, these liquid crystal switches are both expensive and difficult to mass produce.
Specifically, each set of trapezoidal prisms must be precisely matched to ensure the exactness of the optical path, the trapezoidal prisms must also be optically flat to avoid detrimental light scattering at the surface and, in addition, must be optically pure to avoid internal light scattering. Light scattering and optical path inaccuracies can result in both signal loss and crosstalk. Further, the trapezoidal prisms must be precisely aligned to each other to avoid signal losses and crosstalk. Typically, the effect of any single misalignment worsens over the length of the light path thereafter. Finally, the devices require that each trapezoidal prism include four optically flat surfaces to accomodate at least one internal reflection at the prism/air interface opposite the base surface. The requirement for four optically flat surfaces to accomodate the internal reflection severely reduces the allowable tolerances on the positioning of the prisms to each other because of the limited length of the angled surfaces. That is, if the prisms are not precisely aligned to each other the assembly may not be useable since the side angle surfaces are so short that the fibers attached would be misaligned. In addition, such a configuration requires expensive and time consuming surface polishing and lapping processes and a very pure glass. As a result, these optical switches are quite impractical for an extensive optical telecommunication network.
Consequently, since a typical telecommunication network may include hundreds, if not thousands, of switching devices, a low cost, readily mass produced optical switching device is necessary to fully realize the advantages of an optical telecommunications system.